It is known, for example from "Quantum wire lasers", Eli Kapon, Proc IEEE 80 p398-410 (1992), that it is desirable to produce semiconductor devices in which the charge carriers are quantum confined in one dimension in extremely thin active layers (of approximately 20 nm or less), since such devices exhibit improved performance compared to comparable heterostructure devices. Confinement can be arranged to produce an essentially one dimensional structure, called a quantum wire. Structures in which the quantum confinement exists in three dimensions, i.e. the structure is very small in all three spatial dimensions, give rise to quasi-zero-dimensional structures, commonly known as a quantum dot or as a quantum box.
Quantum wire structures have been produced, as reported in "AlGalnP multiple-quantum-wire lasers grown by gas source molecular beam epitaxy", E. M Stellini, K. Y. Cheung, P. J. Pearah, A. C. Chen, A. M. Moy, and K. C. Hsieh, Appl. Phys. Lett 62 p458-460 (1993) and in "Vertically stacked multiple quantum wire semiconductor lasers", S. Simhony, E. Kapon, E. Colas, D. M. Hwang, N. G. Stoffel and P. Worland, Appl. Phys.lett 59, p2225-2227 (1991). However, these papers do not describe a satisfactory method of fabricating such quantum wire structures.
According to a first aspect of the present invention, there is provided a method of making a quantum confined device, comprising forming in a surface of a substrate, a depression having a plurality of walls extending into the substrate from the surface, and modifying the walls by forming a step in each wall so as to define in a base region of the depression a sub-depression of lateral size smaller than that of the depression.
It is thus possible to modify a depression, such as a groove or pit, formed by conventional etching steps to form a region therein which has a relatively small lateral dimension. Such a small dimension effectively provides sufficient dimensional containment to enable essentially one-dimensional, i.e. quantum wire, or zero-dimensional, i.e. quantum dot, structures to be formed therein.
Preferably the depression is formed by etching a semiconductor starting at a {100} surface thereof. The semiconductor may be masked so as to define a groove extending parallel with a &lt;110&gt; crystallographic direction such that the groove walls formed in the etching process are defined by {111} surfaces. This results in the formation of a well defined groove since the {111} surfaces are slow etching surfaces. Alternatively, the etching may be controlled to reveal higher index surfaces, such as a {311} or {511} surface.
In the case of a pit like depression, end surfaces, provided by third and fourth walls which intersect the first and second surfaces of the groove so as to close the groove, may also lie in {111} crystallographic planes.
Preferably a mask is applied to the first and second walls to define regions which are to be substantially protected from the etching. The mask may be applied to the surfaces by evaporative deposition. The mask may be silicon monoxide. Advantageously the upper ends of the walls of the groove have respective overhanging and/or upwardly extending mask regions which cooperate to define a slit. The slit may be used to limit the width of the mask applied to the first and second walls.
Preferably the deposited mask is deposited using a substantially unidirectional stream of atoms or vapour such that the overhanging mask region casts a shadow over part of the walls and thereby prevents the deposited mask being deposited in the shadowed regions. Advantageously the deposited mask for the first wall is deposited in a separate processing step to the deposition of the deposited mask for the second wall.
Preferably the masked first and second walls are anisotropically etched using a chemical etchant. Advantageously a surfactant may be included within the etching mixture so as to enhance wetting of the walls.
In the case of a pit, the mask may also be applied to third and fourth walls which close the groove. Advantageously the overhanging masks at the upper end of the limbs of the pit define an aperture and the overhanging masks are used to cast a shadow on each of the first to fourth walls during the deposition of the deposited mask. The third and fourth walls also undergo anisotropic etching so as to define a pit which may be used to form a quantum dot.
Preferably the masks are removed following the etching step. Silicon oxide masks may be dissolved using hydrofluoric acid.
According to a second aspect of the present invention, there is provided a quantum confined device, comprising a substrate having a depression formed in a surface thereof with a plurality of walls extending into the substrate from the surface, each of the walls having a raised portion defining in a base region of the depression a sub-depression of lateral size smaller than that of the depression.
Preferably the raised portions form respective steps on the walls.
Preferably the first and second raised portions are separated from one another. Thus the raised portions define a groove or a pit of reduced width therebetween.
The presence of the raised portions allows semiconductor layers to be formed within the depression which layers are not continuous over the walls. The discontinuity within the semiconductor layers enables the formation of one dimensional quantum device structures, or in the case of a pit, zero dimensional quantum device structures.